Blade or Owl tome for L!Lyn?

I'd say owl since that can pump up her speed and defenses a little. Refinement probably into defense if you typically have her facing down archers, or Res if someone else handles that.

I'd go owl tome over blade tomes on armor mages, since you'll most likely run Ward/Goad armor due to armor knights usually staying closely packed with their 1 tile movement.

Owl Tome with Spd Refinement to help patch up her spd bane. If your keeping her A skill, then you would be getting +5 Atk/Spd on top of the bonuses from adjacent allies.